Download ASTR2050 Spring 2005 • In this class we will ...

ASTR2050 Spring 2005
Lecture 11am 8 February 2005
In this class we will ...
• Review what we’ve learned about stars
• Study The Sun
1
What we can measure about stars
(1) “Temperature”
More properly “Effective Surface Temperature”
The “old” way: Spectral Classification
i.e. O,B,A,F,G,K,M with numerical sub-classes
The “modern” way: Color Index
Uses color filter bands U,B,V,R,I
(V=“Visual”)
Index based on difference of filtered magnitudes
e.g. mB − mV = MB − MV = B −V
2
(2) “Brightness”
The appropriate physical quantity is “Luminosity”
Relative brightness: Magnitude
m1 − m2 = 2.5 log10( f2/ f1)
Standard brightness: Absolute magnitude
m − M = 2.5 log10(d/10 pc)
2
Convert to luminosity: Bolometric magnitude
MBol (!) − MBol (!) = 2.5 log10 [L(!)/L(!)]
2
4
L
=
(4!R
)("T
)
These are related to stellar radius by
Note:We cannot measure stellar mass or radius directly!
3
Hertzprung-Russell (HR) Diagram
4
Binary Star Systems
Two stars in orbit about their common center of mass
Various types of binary stars.
Doppler shifts give velocity
Orbital mechanics give the
masses of the two stars
Eclipses allow determination
of individual stellar radii
See Studio Laboratory this Friday
5
Example: Sirius (Alpha Canis Majoris)
The brightest star in the sky is actually two stars!
The Orbit
Sirius A: A1 Main Sequence star
SiriusB: White dwarf
Visual
X-ray
Lots of X-rays from Sirius B
6
Luminosity (in solar luminosity)
Example: Masses of stars
“Mass Luminosity Relation”
“Massive stars are
much, much brighter
than lighter stars.”
An important goal of
our study of stellar
structure will be to
understand how this
relation comes about.
Mass (in solar masses)
7
The Sun
A model for studying stars!
The Solar spectrum
Recent photo in H-α
8
Basic Internal Structure of the Sun
9
Doppler Broadening of Spectral Lines
Maxwell-Boltzmann distribution
of molecular velocities
Heat energy realized
as random motion:
1
3
2
m!v " = kT
2
2
This “broadens” spectral
lines by Doppler Shift:
!"
=
"
Speed (m/sec)
10
!
!v2" vrms
≡
c
c
Sunspots: Cooler Regions and Magnetic Fields
11
Solar Rotation
“Differential Rotation”: See Kutner Fig.6.25
12